The present invention relates to a polarizing property electrode for an electrical double layer capacitor and its manufacturing method.
Further, the present invention relates to a manufacturing method of an electrode sheet for an electrical double layer capacitor, and particularly relates to the improvement of a drying process for drying the electrode sheet.
The electrical double layer capacitor has a large capacity of a farad class and is excellent in charge and discharge cycle characteristics. Therefore, the electrical double layer capacitor is used in uses such as a backup power source of an electric device, a battery mounted to a car, etc.
This electrical double layer capacitor is constructed by arranging a pair of polarizing property electrodes, a separator arranged between the pair of polarizing property electrodes, and an electrolyte. The polarizing electrodes separated by the separator are constructed so as to respectively act as an anode and a cathode. Activated carbon having minute holes is normally used as the material of the polarizing electrode of such an electrical double layer capacitor. The electrical double layer capacitor is formed by adsorbing and gathering electrolytic ions within the electrolyte within the minute holes of the activated carbon so that the anode and the cathode of the capacitor are constructed.
The manufacturing method described in the following patent literature 1 is disclosed as the manufacturing method of the above polarizing property electrode. In this manufacturing method, an admixture constructed by activated carbon powder, polymer resin including fluorine and a liquid-shaped lubricant (solvent for lubrication) is molded in a sheet shape. Thereafter, the liquid-shaped lubricant is removed by a means of heating, extraction, etc. This patent literature does not concretely show to which extent the liquid-shaped lubricant is removed by what means.
[Patent Literature 1]
JP-B-7-105316 (Japanese Patent Publication (KOKOKU) No. 7-105316)
Further, the electrical double layer capacitor has a large capacity of a farad class, and is excellent in charge and discharge cycle characteristics. Therefore, the electrical double layer capacitor is used as a backup power source of an electric device and a battery of each of various kinds of transports such as an automobile. Further, it is also considered that the electrical double layer capacitor is used in a use such as the storage of electric power in the nighttime in the future from the viewpoint of energy effective utilization.
The structure of the electrical double layer capacitor and the charging and discharging mechanism will be briefly explained. First, in this structure, a polarizing property electrode for the electrical double layer capacitor is dipped into an ionic property solution. This polarizing property electrode is constructed by a sheet (hereinafter called an electrode sheet) nipping a current collecting foil by two sheet-shaped activated carbons (sheet-shaped electrodes), and an insulating separator (hereinafter called a separator) permeating ions thereinto. In the charging and discharging mechanism, a voltage is applied to the above polarizing property electrode for the electrical double layer capacitor to such an extent that no electrolysis is caused. The charging and discharging operations are performed by opposing plus and minus electricities at the interface.
Accordingly, when moisture is left in the electrode sheet and the voltage is applied, its moisture is electrolyzed in the above nature of the electrical double layer capacitor and there is a fear that a reduction in performance of the electrical double layer capacitor is caused. Therefore, in a manufacture process, a drying process for removing the moisture of the electrode sheet to such an extent that there is no practical problem, is required.
For example, there is the following method as a manufacture method of the electrical double layer capacitor including the drying process with the moisture removal as an object. Namely, in this method, the polarizing property electrode for the electrical double layer capacitor constructed by the electrode sheet and the separator is stored to a cylindrical container having a bottom as a container for the electrical double layer capacitor. Thereafter, the container is sealed by a cover body having a liquid injecting port. Next, the entire container is dried in an atmosphere having a dew point of −20° C. or less. A non-water based electrolyte is then injected from the above liquid injecting port, and this liquid injecting port is sealed (for example, see patent literature 2).
Further, for example, there is also a method in which a raw material of the sheet-shaped electrode constructed by carbon based powder, an electrically conductive assistant and a binder is mixed and kneaded and the drying process of moisture removal is performed with respect to this kneaded object (for example, see patent literature 3).
[Patent Literature 2]
JP-A-2000-49052 (pp. 3–4)
[Patent Literature 3]
JP-A-2001-307964 (pp. 4–5)
The activated carbon used in the electrical double layer capacitor originally has a property excellent in the adsorbing ability of a chemical substance. This adsorbing ability is originated in the existence of the minute hole of the above activated carbon. Accordingly, when the liquid-shaped lubricant is not sufficiently removed and is left within the minute hole, problems exist in that the formation of the electrical double layer capacitor using the adsorption and the gathering of the electrolytic ions is prevented and the internal resistance of the electrical double layer capacitor is increased.
Further, as mentioned above, when the moisture is mixed into the electrode sheet in applying the voltage to the polarizing property electrode for the electrical double layer capacitor, this moisture causes the electrolysis. Therefore, the problems of a reduction in electrostatic capacity of the electrical double layer capacitor, a rise in resistance, deterioration of durability, etc. are caused.
However, since the activated carbon (carbonaceous powder) constituting the electrode sheet is porous, the adsorbing property of the moisture is very high. Moreover, when the respective raw materials are mixed, etc., the organic solvent of an alcohol system is added to facilitate the mixture. Therefore, a reciprocal problem exists in that the moisture inevitably exists in the raw materials.
Accordingly, the drying process for setting the remaining amounts of the moisture and the organic solvent in the electrode sheet in the manufacture process to very low levels in its manufacture process is indispensable to the security of durability and reliability of the electrical double layer capacitor for a long period.
However, in the above patent literature 2, the polarizing property electrode for the electrical double layer capacitor constructed by the electrode sheet and the separator is dried in a cylindrical winding state within the container. Therefore, it is difficult to preferably dry the polarizing property electrode for the electrical double layer capacitor until its central portion.
Further, in the above patent literature 3, the drying operation is performed at a stage at which the raw material of the sheet-shaped electrode is mixed and kneaded instead of the electrode sheet. Therefore, a problem exists in that the sheet-shaped electrode again absorbs the moisture after that until the kneaded object is rolled and the sheet-shaped electrode is manufactured and is further completed as the electrode sheet by sticking this sheet-shaped electrode to a foil.